1. Field of the Invention
The present invention relates to an optical pickup apparatus.
2. Description of the Related Art
As a light source of an optical pickup apparatus that reproduces an optical information recording medium such as a CD (Compact Disc) and DVD (Digital Versatile Disc), a semiconductor laser element is generally used. The wavelength of a laser beam as used and the number of apertures (NA) of the objective lens as used differ depending on whether a recording medium to be reproduced is a CD or DVD. For example, the wavelength of the laser beam is 650 nm and NA is 0.6 for a DVD. On the other hand, the wavelength of the laser beam is 780 nm and NA is 0.45 for a CD.
In order to allow a single player to reproduce both CD and DVD, an optical pickup apparatus incorporating a light source with two wavelengths of 650 nm and 780 nm has been under development and a semiconductor laser unit for emitting laser beams having two different wavelengths has been used in recent years in particular. This type of semiconductor laser unit includes a laser diode with a wavelength of 780 nm for a CD and a laser diode with a wavelength of 650 nm for a DVD, each of which is mounted in a single package.
When an optical pickup apparatus is constructed using the above described semiconductor laser unit with two wavelengths, the positional relationship between the central axis (optical axis) of the optical system extending from the light source to the disc and the light source is an important issue. Basically, placing the light source on the optical axis makes it possible to control occurrence of various types of aberration such as coma aberration. However, since the above described semiconductor laser unit with two wavelengths has two light sources for CD and DVD, it is physically impossible to place both lights sources on the optical axis. For this reason, many optical pickup apparatuses using a double-wavelength semiconductor laser unit include a CD light source provided on the optical axis and a DVD light source provided at a position shifted from the optical axis accordingly.
However, if the DVD light source is placed in a position shifted from the optical axis, the laser beam emitted from the DVD light source has a height of an image relative to the objective lens, thus causing coma aberration. Furthermore, the amount of coma aberration generated also varies depending on the design of the objective lens used, and therefore depending on the size of the objective lens used, coma aberration associated with the shape and position of the objective lens occurs in addition to coma aberration caused by an image height of the laser beam for DVD. As a result, the total amount of coma aberration grows considerably and deteriorates the optical characteristic of the optical pickup apparatus. This is an example of the problems to be solved by the present invention.